The invention relates to a method and a wire electrode for the spark-erosive cutting of hard metal comprising a core and at least one outer coating surrounding the core and wearing during cutting.
The term hard metal identifies a metal ceramic compound, for example a WC/Co-compound, which has as a characteristic a great hardness and a high resistance to wear, in particular at high temperatures. The hard metal consists of carbides, which are held with a binder, for example Co, Ni, in a structure. The binder can be up to 35% by weight. The binder is mainly needed in order to bond ceramic components like carbides, and lend to the hard metal the necessary toughness during mechanical loads. The carbides, which give the hard metal its extreme hardness and resistance ability, are ceramic-like materials. They have a high resistance to corrosion but, however, are very sensitive to breakage.
For the manufacture of highly precise tools or matrixes it is necessary to work hard metal blocks by means of common methods, for example by grinding or spark erosion. The spark erosion has thereby lately been more and more successful because of its great precision, its versatility and its high degree for automation, in connection with advantageous production costs.
However, it is problematic for the spark-erosive cutting of hard metals that among others pitting phenomena occurs on the not eroded surfaces of the hard metal block. This pitting consists of many small holes on or directly below the surface of the hard metal block. This pitting cannot always be recognized with the naked eye, and the surface of the hard metal block can appear on the outside as being completely sound. However, as soon as the questionable area is subjected to stress due to wear, the previously covered-up holes appear. The holes are created by the corrosion of hard metal This is an electrochemical process which attacks the entire surface of the block dipped into the watery dielectric. The corrosion causes a washing out of the metallic phase of the hard metal, connected with a freeing of the metal contained in the hard metal in the dielectric.
The electrochemical corrosion of hard metals is expressed in a destruction of the hard metal structure, which destruction starts out from the surface. It is based on a dissolving of the binder contained in the hard metal and, if applicable, further metals, The cause is hereby the presence of a second, nobler metal, the standard electrode potential of which is thus higher than the one of the corroding metal of the hard metal. The electromechanical corrosion takes thereby place on the anode of an electrochemical corrosion cell and occurs In the presence of an electrolyte (dielectricum) These conditions exist during the spark-erosive cutting of hard metal.
The basic purpose of the invention is to provide a wire electrode with which the corrosion, in particular of the not eroded surfaces of the hard metal block is prevented.